1. Field of the Invention
The present invention relates to a planar light source device using point light sources and in particular to a planar light source device applicable to a display device using a liquid crystal.
2. Description of the Related Art
A liquid crystal display device, a signboard or a guide lamp is a non-luminous display device rather than a luminous display device including a luminous display part. Such a non-luminous display device requires a planar light source device such as a backlight on the rear surface of a display part. Planar light source devices come in side light type (edge light type) or direct backlight type. The side light type planar light source device has a light source arranged on the side surface of a housing case while the direct type planar light source device has a light source arranged on the rear surface of a housing case opposed to a liquid crystal display part. Some of the side light type planar light source devices use a light guide plate for guiding light from a light source in a desired direction. A planar light source device using a light guide plate reflects light emitted from a side light part composed of a linear light source such as a CCFL (Cold Cathode Fluorescent Lamp) or a point light source such as an LED (Light Emitting Diode) inside a light guide plate and causes the light to be diffused in a dot pattern provided inside the light guide plate thus emitting the light in a planar shape.
In general, for a planar light source device using a point light source such as an LED as a light-emitting element, in order to enhance the luminance of a display screen, the number of light-emitting elements is increased to raise the density of elements or increase the current value to be supplied to each point light source. In any way, heat generated by each point light source that accompanies lighting causes the periphery of the point light source to be excessively hot. This lowers the luminous flux of the point light source thus shortening the service life of the point light source and reducing reliability. A planar light source device has been proposed including heat dissipation means on a substrate including point light sources (for example, refer to JP-A-2002-229022 or JP-A-2003-76287). In such a planar light source device, the heat dissipation is improved so that it is possible to increase the number of elements of point light sources and the current supplied to each point light source.
In case a point light source is used as a light source in a related art planar light source device, a plurality of light-emitting diodes are arranged in a row to form a light source. This results in unevenness in luminance between the proximity to a point light source and the proximity to the area between adjacent point light sources. Thus, in other related art planar light source devices, pits and projections are provided on the light incident surface of a light guide plate and the light-emitting diodes are fitted into the pits and reflecting members are provided on the projections on the light incident surface of the light guide plate. With this configuration, unevenness in luminance of the light emitted from an opening has been reduced (for example, refer to JP-2001-14922).